Callisto Moon

Callisto
Photo of Callisto taken by Voyager 2 .
Orbital characteristics ( Epoch 16 January 1997
Periapsis	1,869,000 km
Eccentricity	0.0074
Period of revolution	16.6890184 d
Tilt	0.192 (Relative to the plane Jupiter's equatorial)
Physical Characteristics
Diameter	4 820.3 3.0 km
Mass	1.075938 0.000137 10 23 kg
Density medium	1.834 4 0.003 10 3 kg / m
Gravity at the surface	1.235 m / s 2
Rotation period	Synchronous rotation of
Albedo using	0.22 (geometric)
Temperature surface	165 5 (max) 134 11 (mean) 80 5 (min) K
Characteristics of the atmosphere
Discovery
Discovered by	Galileo
Discovery	7 January 1610
Designation (s) provisional (s)	Callisto, J IV Callisto, Jupiter IV
change

Callisto (J IV Callisto) is a natural satellite of the planet Jupiter , discovered in 1610 by Galileo . Callisto is the third largest moon in the solar system , the second of the Jovian system, after Ganymede. It is also the Galilean moon farthest from Jupiter and the only one not to be in orbital resonance. Callisto may have formed by accretion disk of gas surrounding Jupiter after its formation .

Callisto is composed of approximately equal parts of rock and ice. Due to the lack of heating due to tidal forces , the moon would be only partially differentiated. Research conducted using the Galileo spacecraft revealed that Callisto may have a small nucleus composed of silicates and an ocean of water liquid at over 100 kilometers below the surface of the moon ^, which could accommodate the extra-terrestrial life .

Callisto's surface is cratered and extremely old and shows no sign of tectonic activity . Callisto's surface is less affected by the magnetosphere of Jupiter as the other inner moons since it is furthest from the planet . Callisto is surrounded by an atmosphere made up mainly of very thin carbon dioxide and probably molecular oxygen , as well as an ionosphere intense .

Several space probes , the Pioneer 10 - 11 to Galileo and Cassini have studied the moon. Callisto has long been regarded as the body best suited to the installation of a base for human exploration of the Jovian system .

Discovery

Callisto was discovered by Galileo in January 1610, at the same time as the other three large moons of Jupiter , Ganymede , Io and Europe . However, it is possible that Gan De , a Chinese astronomer, have observed the in 362 BC. J.-C . Callisto is named Callisto , one of many conquests of Zeus in Greek mythology. This name was suggested by Simon Marius soon after the discovery . According to Marius, the name would have in fact been suggested by Johannes Kepler . However, the Roman numeral designation introduced by Galileo is used until the mid- ^twentieth century at the expense of the names of moons from mythology. Under this designation, and Callisto is called Jupiter IV because it is the fourth satellite of Jupiter .

The remarkable geological formations of Callisto have been named after the Norse mythology. Thus, the two largest craters are named Valhalla (heaven fallen warriors) and Asgard (place of residence of the gods). The other craters are named names of heroes: Valfodr , Hoenir , Lodurr , Bran , Sudri , Fodri , NIDI , Burr , Reginn , Ymir , Gymir , etc..

Orbit and rotation

Size comparison of the Moon (top left), Callisto (lower left) and Earth (right). Credit: NASA / JPL

Galilean moon Callisto is the furthest from Jupiter. It orbits the planet at a distance of 1 880 000 km (26.3 times the radius of Jupiter) . The radius of its orbit is much larger than the second outermost moon, Ganymede , the radius of the orbit is 1.07 million km. Callisto is much more distant than the other three moons, it is not in orbital resonance with them and did probably never been .

Like many planets moons, Callisto is in synchronous rotation around Jupiter . Day length, identical to its orbital period is approximately 16.7 Earth days. Its orbit is slightly eccentric and inclined to the Jovian equator. Its orbital inclination and eccentricity are almost periodic (on the scale of centuries) because of gravitational perturbations from the Sun and Jupiter. The amplitude variations, respectively from 0.0072 to 0.0076 and 0.20 - 0.6 . These orbital variations are the cause of changes in the tilt of its axis (the angle between the axis of rotation and the orbital plane) of amplitudes between 0.4 and 1.6 .

Because of its distance from Jupiter, the moon has never been significantly heated by tidal forces , which has important consequences on its internal structure and its evolution . Similarly, the flow of charged particles from the magnetosphere of Jupiter is relatively low at the surface of Callisto: it is 300 times lower than that received by the area of Europe. Unlike the three other Galilean moons, irradiation by charged particles had little effect on the surface of Callisto .

Physical characteristics

Composition

Spectrum in the infrared near an area of cratered plains. Credit: NASA / JPL-Caltech

The density of Callisto average, 1.83 g cm ^-3 , suggests that the moon is made of rock and ice water in roughly equal proportions, plus a few frozen volatiles such as ' ammonia . The mass fraction of ice is between 49 and 55% ^, . The exact composition of the rocks of Callisto is unknown but is probably close to the composition of ordinary chondrite type L / LL, which are characterized by a smaller proportion of total iron and iron in metallic form, but more of oxide iron in the H-type chondrite. The mass ratio iron / silicon is between 0.9 and 1.3 on Callisto cons about 1.8 for the Sun .

Callisto's surface has an albedo of about 20% . The composition of its surface would be representative of its overall composition. Work spectroscopic conducted in the near infrared showed the presence of absorption lines due to water ice at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers . Water ice seems to have an isotropic distribution on the surface of which she composed between 25 and 50 wt% . Analysis of high resolution spectra in the near infrared and ultraviolet taken by Galileo and the Earth have been identified material other than ice, such as silicates, hydrous iron and magnesium , of carbon dioxide , the sulfur dioxide and perhaps of the ammonia and other organic compounds ^, . The spectral data indicate that the surface is extremely heterogeneous on a small scale. Small bright spots composed of pure water ice is mixed with patches of rock-ice mixture and large dark areas of materials, not frozen ^, .

Callisto's surface is asymmetric: the leading hemisphere (that facing the direction of orbital motion) is darker than the trailing hemisphere. Other moons have the reverse situation . The trailing hemisphere of Callisto is enriched with carbon dioxide , while the atmosphere before has more sulfur dioxide . Many impact craters as young lofn have a higher concentration of carbon dioxide in them or near them . According to Greeley et al, the chemical composition of the surface could be broadly similar to that of D-type asteroids , whose surface is composed of carbonaceous materials.

Internal Structure

Internal structure of Callisto. Credit: NASA / JPL-Caltech

Callisto is covered with a lithosphere thickness of ice between 80 and 150 km ^, . A salty ocean could be located under the crust ^, , as suggested by studies on the magnetic field around Jupiter and its moons ^, . Callisto behaves like a perfect sphere conductor in the magnetic field of Jupiter, in other words, the field does not penetrate inside the moon, suggesting that Callisto would have within it a highly conductive fluid whose minimum thickness is 10 km . The probability of existence of an ocean is enhanced if the water contains a small amount of ammonia or another compound antifreeze in a mass ratio less than or equal to 5% . In this case, the ocean may have a thickness of up to 250-300 km . If Callisto proves devoid of ocean, its lithosphere may be thicker than today considered and measured up to 300 km.

Below the lithosphere and the ocean within Callisto would not be very homogeneous nor completely heterogeneous. The data obtained by Galileo , especially the moment of inertia dimensionless , 0.3549 0.0042 calculated at close fly-bys, suggest that the interior is composed of rocks and ice compressed. The proportion of rocks increase with depth due to a partial separation of its components due to their different densities. In other words, Callisto is only partially differentiated. Its density and its moment of inertia are compatible with the existence of a small heart in the center of silicates of the satellite. The radius of such a heart is less than 600 km and its density between 3.1 and 3.6 g cm ^{-3 ,} .

Surface

Cratered plains. Credit: NASA / JPL-Caltech

The ancient surface of Callisto is one of the most heavily cratered in the solar system . In fact, the density of impact craters is close to saturation: new crater will tend to remove an old one. On a larger scale, the geology is relatively simple: the planet does not have mountains, volcanoes or other geological feature of tectonic origin endogenous . Impact craters and multi-ring structures - associated with fractures, fault scarps and deposits - are the only major geological features to be present on the surface ^, .

Callisto's surface may be divided into different geological areas: cratered plains, plains clear, smooth plains of lustrous and units associated with multi-ring structures and impact craters ^, . The cratered plains constitute most of the surface of the moon and represent the old lithosphere , a mixture of ice and rock materials. The plains include bright bright impact craters as Burr and lofn and erased the remnants of old craters called palimpsest , the central portions of multi-ring structures, and isolated patches in the cratered plains . These plains are clear of icy impact deposits. The appearance of bright smooth plains constitute a small fraction of Callisto's surface and are present in areas of ridges and fractures, craters Valhalla and Asgard and sometimes in the cratered plains. Scientists thought they were related to the endogenous activity but high resolution images from the Galileo spacecraft showed that the smooth plains of glossy appearance were related to land very rough and fractured and show no signs of resurfacing . The Galileo images have revealed small areas smooth and dark with a total area of less than 10,000 sq. km. Those might be deposits cryovolcanic . Plains clear and smooth plains are younger and less cratered than the background cratered plains ^, .

Har impact crater and its central dome. Credit: NASA / JPL-Caltech

The diameter impact craters observed on Callisto is 0.1 km, the limit of image resolution, more than 100 miles, not counting the multi-ring structures . Small craters whose diameter is less than 5 km, are simple bowl-shaped depressions or flat bottom. With a diameter of between 5 and 40 km in general have a central peak. Larger craters (diameter between 25 and 100 km), as the crater Tindra, have central depressions instead of peaks . Large craters with diameters greater than 60 km, such Doh and Har, can have central domes, which are due to tectonic uplift after impact . A small number of very large craters with diameters brilliant than 100 km exhibit abnormal structures in domes. They generally have a low height and geomorphological structures could be transitional to the multi-ring structures, like the crater lofn . Callisto's craters are generally shallower than the Moon.

Valhalla ring structure. Credit: NASA / JPL-Caltech

The largest impact structures on the surface of Callisto are multi-ring basins ^, , two of which have a size unusual. The crater Valhalla is the biggest, with a bright central region with a diameter of 600 km, and rings extending to 1800 km from center (see Figure) . The second is the size Asgard with a diameter of 1 600 km . These multi-ring structures are probably the result of a concentric fracturing of the lithosphere after the impact. The lithosphere was based on a layer of ductile materials or liquids, may be an ocean . The catenae example, the catena Gomul, are long chains of impact craters lined up at the surface of Callisto. They are probably due to objects that were disrupted by tidal forces during a close passage of Jupiter and who then crashed on Callisto, or by very oblique impacts . The comet Shoemaker-Levy 9 is an example of such a body that was broken into pieces by Jupiter.

As mentioned previously, small patches of pure ice of albedo up to 80% were found on the surface of Callisto, and are surrounded by darker parts . High resolution images taken by Galileo show that the bright spots are mainly located on elevated areas, the raised rims of craters, scarps, ridges and bumps and knobs. These are probably thin deposits of ice. The dark materials are usually located in low areas surrounding the parties would be rather flat and shiny. They often form patches up to 5 km across the bottom of craters and depressions between the craters .

Landslides and small spots. Credit: NASA / JPL-Caltech

At the scale of kilometers, the surface of Callisto is more degraded than surfaces of other moons . It has a small deficit of impact craters with diameters less than 1 km compared to dark plains of Ganymede . Instead of small craters, the surface of Callisto present almost everywhere in small bumps and depressions . The rough edges are the remains elevated craters that have been broken by an unknown process . The process is most likely the slow sublimation of ice, which occurs at temperatures up to 165 K , reached in the regions of Callisto where the sun is at its highest . This process of sublimation of water or other frozen volatiles is responsible for the decomposition of dirty snow (the rock) from which they come. The materials are not frozen avalanches of debris that descend the slopes of the crater walls . Such avalanches are regularly observed near or within impact craters and are called 'debris aprons, ^{, ,} . Occasionally, the walls of the craters are not cut incisions resembling winding valleys and called 'Gull', which resemble some surfaces observed on Mars . If the hypothesis is confirmed by sublimation, the dark materials in low heights are primarily a film of debris not frozen, which are caused by raised rims of craters that have deteriorated and have covered a rock mostly ice.

The relative ages of different regions of Callisto can be determined from the density of impact craters they behave. More the surface is old, the more craters it has a high . No absolute dating has been conducted, but based on these theoretical considerations, age plains crater gusts were estimated at about 4.5 billion years, roughly the time of the formation of the solar system. The age structures and multi-ring impact craters depend on the rate of cratering of the surface studied and their age is estimated by various authors between 1 and 4 billion years .

Atmosphere and ionosphere

Magnetic field around Callisto. The shape of field lines indicates the existence of an electrically conductive layer inside Callisto. The red line shows the trajectory of the Galileo spacecraft during a fly-by typical (C3 or C10).

Callisto has an atmosphere tenuous, consisting mainly of carbon dioxide . It was discovered in the Near Infrared Mapping Spectrometer Spectrometer (NIMS) of Galileo: Researchers have identified the absorption line at 4.2 micrometers CO _2. The pressure at the surface is estimated at 7.5 10 ^-12bar and the particle density to 4 10 ⁸ cm ^-3. The atmosphere is probably fed continuously for so tenuous atmosphere would disappear within a few days otherwise. It could be fueled by slow sublimation of carbon dioxide ice in the icy crust of the satellite , a process that would be consistent with the hypothesis of formation of small bumps shiny surface by sublimation.

The ionosphere of Callisto was detected for the first time during one of fly-bys (overflights) of the Galileo probe ; the ionosphere has a high density of electrons (7-17 10 ⁴ cm ^-3) can not be explained solely by the process of photoionization of atmospheric carbon dioxide. Thus, the atmosphere of Callisto could be primarily composed of molecular oxygen , which would amount to ten to one hundred times greater than CO ₂ . However, there is no direct evidence of the presence of oxygen in the atmosphere of Callisto. The observations made with the Hubble Space Telescope (HST) have established an upper limit to the concentration of oxygen in the atmosphere, based on the lack of detection of the element by Hubble, which is consistent with the measures the ionosphere of the moon . At the same time, Hubble has detected oxygen condensed trapped on the surface of Callisto .

Disc debris Callisto

In 1999, a debris disk in the form of a ring has been detected around Callisto as well as for Europe and Ganymede .

Origin and Evolution

Rough terrain. Credit: NASA / JPL / Arizona State University

The partial differentiation of Callisto is to say, the partial separation of different materials depending on their density is deduced from measurements of the moment of inertia, states that the moon has never been sufficiently heated to melt the ice . Therefore, the most likely model for training is a slow accretion in subnebula low density of Jupiter (a disk of gas and dust located around Jupiter after its formation) . Such accretion phase would allow cooling to contain the increased heat caused by the impacts, radioactivity and compression, thereby preventing the fusion of materials and rapid differentiation . The formation of Callisto lasted between 0.1 and 10 million years .

The subsequent evolution of Callisto after the phase of accretion was marked by the phenomena of heating due to radioactivity, cooling by thermal conduction near the surface and convection in the solid state or subsolidus within . The state subsolidus convection in ice is the biggest unknown in the models for icy moons. This phenomenon appears when the temperature is sufficiently close to the melting point , due to the temperature dependence of the viscosity of the ice . The state subsolidus convection in icy bodies is a slow movement with ice about 1 cm / year, but it is a very significant cooling process over long time scales (geological scales ) . The phenomenon at work would be a stagnant-lid regime, where the cold and rigid outer layer of the moon removes heat without convection, while convective processes occur in the ice below the subsolidus state. For Callisto, the outer layer is conductive to the cold and rigid lithosphere whose thickness is a hundred miles. Its presence would explain the absence at the surface of Callisto, signs of endogenous activity ^, . The ice layer in which convection occurs could consist of several sub-layers, because under the high pressures that have effect, the water ice exists in different crystalline forms of ice I to the surface to the ice VII at the center . The process of subsolidus convection within Callisto could have prevented (if started early in the history of the moon) the melting of ice on a large scale and differentiation that would have otherwise formed a large nucleus rocky and icy mantle. Due to the convective process, separation and differentiation of slow and partial rock and ice within Callisto has however taken place in all of billion years and may continue today .

According to current theories on the evolution of Callisto, the moon would have within it a layer, an ocean of liquid water. Its existence is linked to particular behavior of the melting of the crystalline form I of the ice , the temperature decreases with pressure, reaching 251 K (about -22 C ) to 2070 bar . In all realistic models of Callisto, the temperature of the layer between 100 and 200 km depth is very near or slightly above this melting unusual ^{, ,} . The presence of even small amounts of ammonia (about 1 or 2 wt%) virtually guarantee the existence of a liquid phase because the melting temperature of the mixture would be even lower .

Although the overall composition of Callisto is very similar to Ganymede , it would have been a geological history is much simpler. The surface is mainly formed under the influence of impacts and other exogenous factors . Unlike its neighbor, Ganymede whose land is crossed by paths that can reach several hundreds of kilometers long, there is little evidence of tectonic activity on Callisto . The relatively simple geological history of Callisto allows planetary scientists to use the moon as a reference for the study of more complex objects .

Presence of life in the oceans

As for Europe and Ganymede , some scientists have speculated that extraterrestrial microbial life forms can exist in a hypothetical salty ocean beneath the surface of Callisto . However, conditions are much less favorable to the emergence of life on Callisto as Europe, due to possible lack of contact between the ocean and the rocky core (which would prevent the presence of hydrothermal Mountains and the lower dissipating heat flux of the inner layers of Callisto . The scientist Torrence Johnson said about the probability of existence of life on Callisto compared to the other Galilean moons:

"The basic ingredients of life - what we call" pre-biotic chemistry '- are abundant in many solar system objects such as comets, asteroids and icy moons. Biologists believe liquid water and energy needed to support life, and it is therefore interesting to find a place could be liquid water. But on energy, the ocean of Callisto is only heated by radioactive elements, while Europe also has the power generated by the tidal forces due to its greater proximity to Jupiter . "

Based on the considerations already mentioned and other scientific work, Europe is the Galilean moon with the greatest chance of harboring microbial life forms ^, .

Exploration

Artist view of a hypothetical human base on Callisto

Probes Pioneer 10 and Pioneer 11 , which passed close to Jupiter in the year 1970 provided little new information about Callisto compared to observations made from Earth . The real breakthrough corresponded to overflights of Voyager 1 and 2 in 1979-1980. They photographed over half the surface of Callisto with a resolution of 1-2 km and measured each specifically its temperature, mass and shape . It took however until the Jovian exploration probe Galileo for that new discoveries are made. From 1994 to 2003, Galileo flew eight times Callisto. During his last visit in 2001, during the C30 orbit, it passed only 138 km from the surface. Galileo finished photographing the entire surface of Callisto and took pictures of parts of Callisto with a resolution of up to 15 meters . In 2000, the spacecraft Cassini-Huygens during its journey to Saturn , provides high-resolution infrared spectra of the Galilean moons, Callisto including . In February-March 2007, the spacecraft New Horizons whose final destination is Pluto made new images and spectra of Callisto .

In 2003, NASA conducted a theoretical study called Human Outer Planets Exploration (HOPE, "Human Exploration of Outer Planets") on the human exploration of the outer solar system. The study was particularly interested in Callisto . It was proposed to build a base on the surface of Callisto that would produce fuel to conduct the further exploration of the rest of the solar system . The choice was Callisto because she suffers from low radiation and its geological stability. Basis there would subsequently explore Europe and could serve service station for vessels going Jovian explore the outermost regions of the solar system . These ships would carry a low altitude flyby of Jupiter Callisto after leaving to use the gravity assist from that planet to propel themselves .

Notes

Related articles

• Jupiter
• Europe
• Io
• Ganymede
• Galileo

Source

• (In) This article is partially or entirely from the article in English entitled " Callisto (moon) "(see the list of authors )

References

Version of March 2, 2008 This article has been recognized as "quality item, that is to say that it meets quality standards for style, clarity, relevance, citation of sources and illustration.

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